The common catfish, Ictalurus punctatus, and the protozoan parasite, Ichthyophthirius multifiliis, provide a unique and useful host/parasite system for the study of cutaneous mucosal immunity. Our long term objective is to elucidate the hosts's mucosal immune response against this virulent pathogen. We wish to test the hypothesis that the channel catfish possesses a secretory immune system in the skin which is independent yet closely associated with the systemic lymphoid tissues. We propose that the skin and gill are primary sites of antigen processing as well as target sites for immune cells generated either locally or in the central lymphoid organs (i.e., head kidney and spleen). The specific aims of this proposal are: 1) to determine by ELISPOT the numbers of antibody secreting cells (ASC) in catfish skin, gill, head kidney and spleen over time following defined routes of I. multifiliis exposure (i.e., skin, gill and peritoneum); 2) to identify and localize I. multifiliis specific ASC in the gill and skin using in situ immunohistochemical techniques following defined parasite exposure; 3) to determine by ELISA antibody titers in catfish sera and cutaneous mucus following surface or intraperitoneal infection and 4) to correlate ELISA and ELISPOT data with I. multifiliis immunity. Characterizing the sequential development of parasite-specific ASC in the catfish will shed light on the existence of a piscine mucosal immune system, thus leading to studies defining the mechanism of communication between antibody induction and effector sites, as well as an understanding of antibody- based parasite resistance. In addition, elucidating the ontogeny of the catfish mucosal immune response will complement an on-going effort in the mentor's laboratory to develop a vaccine against I. multifiliis.